Machine and method for monitoring the state of a safety bearing of a machine

ABSTRACT

In a method for monitoring a state of a safety bearing of a rotor shaft of a machine, with the rotor shaft being supported by a magnetic bearing and the safety bearing having an outer ring and an inner ring arranged for rotation with respect to the outer ring, the rotor shaft of the machine is caught with the safety bearing when the magnetic bearing of the machine fails. The magnetic bearing is witched off for monitoring the state of the safety bearing. The rotor shaft is rotated with the machine under control of a higher-ranking controller by using a defined motion sequence, and a physical variable of the safety bearing is measured with a sensor.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of prior filed copending U.S.application Ser. No. 13/322,701, filed Feb. 13, 2012, the priority ofwhich is hereby claimed under 35 U.S.C. §120, and which is the U.S.National Stage of International Application No. PCT/EP2010/055046, filedApr. 16, 2010, which designated the United States and has been publishedas International Publication No. WO 2010/136264 and which claims thepriority of German Patent Application, Serial No. 10 2009 022 835.7,filed May 27, 2009, pursuant to 35 U.S.C. 119(a)-(d).

The contents of U.S. application Ser. No. 13/322,701, InternationalApplication No. PCT/EP2010/055046, and German Patent Application, SerialNo. 10 2009 022 835.7 are incorporated herein by reference in theirentireties as if fully set forth herein.

BACKGROUND OF THE INVENTION

The invention relates to a method for monitoring the state of a safetybearing of a machine. The invention also relates to a relevant machine.

To support the rotating rotor shaft in a machine magnetic bearings,which hold the rotating rotor shaft in a floating state, are being usedever more frequently to support the rotor shaft during operation. If themagnetic bearing fails, as a result of a power failure for example, therotor shaft falls into a safety bearing and is caught by the latter. Thesafety bearing thus serves to catch the rotor shaft. The safety bearingtemporarily takes over the support of the rotor shaft until the rotorshaft has come to a complete standstill. Safety bearings must on the onehand resist the impact when the rotating shaft drops down into thesafety bearing and on the other hand guarantee that the rotor shaftcoasts safely to a halt. For this purpose the bearing ring of the safetybearing has a slightly larger internal diameter compared to the rotorshaft diameter, so that in normal operation, i.e. when the magneticbearing is active, the rotor shaft does not touch the safety bearing.Usually the safety bearing is accommodated in the stator housing in thearea of the respective end of the rotor shaft.

When catching the rotor shaft the safety bearing will be subjected toconsiderable stresses which lead to wear on the safety bearing. The lifeof the safety bearing is shortened by the wear with, in the worst casethe safety bearing, because of the heavy wear which can even occurduring a single runout of the rotor shaft into the safety bearing, nolonger being capable of being used for a further crash even after asingle crash of the shaft into the safety bearing. The safety bearingsare built into the machine and as a rule cannot be inspected therewithout dismantling parts of the machine. The optimum moment forreplacing a defective or deteriorating safety bearing thus cannot beestablished with any certainty. The problem has been solved previouslyby counting the crashes. If a predefined number of crashes of the rotorshaft into the safety bearing is exceeded, e.g. for five crashes, thesafety bearing must be replaced. The safety bearing can however alreadybe worn out after fewer than five crashes or also stand up tosignificantly more crashes. The first case can result in failure of thesafety bearing, the second case in unnecessary and expensive downtimesof the machine in order to replace safety bearings that are not yetdefective or worn.

SUMMARY OF THE INVENTION

The object of the invention is to make it possible to monitor the stateof a safety bearing built into a machine.

The object is achieved by a method for monitoring the state of thesafety bearing of the machine, wherein the safety bearing catches arotor shaft of the machine on failure of a magnetic bearing of themachine, wherein the safety bearing has an outer ring and an inner ringarranged rotatably in relation to the outer ring, wherein the magneticbearing is switched off, wherein the rotor shaft is rotationally movedwith a defined course of movement, wherein a physical variable of thesafety bearing is measured by means of a sensor.

This object is also achieved by a machine, wherein the machine has amagnetic bearing and a safety bearing, wherein the safety bearingcatches a rotor shaft of the machine on failure of a magnetic bearing ofthe machine, wherein the safety bearing has an outer ring and an innerring arranged rotatably in relation to the outer ring, wherein themagnetic bearing is switched off, wherein the rotor shaft isrotationally moved with a defined course of movement, wherein themachine has a sensor by which a physical variable of the safety bearingis able to be measured.

The invention makes it possible to monitor the state of the safetybearing and to recognize whether the safety bearing must be replaced asa result of too much wear.

Advantageous embodiments of the invention are set forth in the dependentclaims.

Advantageous embodiments of the method emerge in a similar way to theadvantageous embodiments of the machine and vice versa.

It proves to be an advantage for the physical variable or for a variablederived from the physical variable to be compared to a target variableand if the deviation of the physical variable or of the derived variablefrom the target variable exceeds the threshold value, for a warningmessage to be generated. This makes it possible to automatically detecta worn safety bearing and a user of the machine or service personnel forexample are automatically informed if the safety bearing is worn.

It also proves to be an advantage for a safety bearing carrier to bearranged around the outer ring for attaching the safety bearing in themachine, with the sensor being arranged on the side of the outer ringfacing towards the safety bearing carrier. The physical variable can bemeasured especially well at this point.

It also proves to be an advantage for the physical variable to beavailable in the form of the temperature of the safety bearing or in theform of a force occurring between outer ring and the safety bearingcarrier or in the form of oscillations of the safety bearing or in theform of a pressure occurring between outer ring and the safety bearingcarrier. Temperature, force, pressure or oscillations represent normalphysical variables of a safety bearing which change as the wear on thesafety bearing increases.

It also proves to be an advantage for the sensor to be arranged betweenouter ring and safety bearing carrier since the force transmitted fromthe safety bearing to the safety bearing carrier can then be determinedespecially well.

It also proves to be an advantage for the sensor to be embodied flat andembedded in a film or arranged on a film, since then the sensor can bebuilt into the machine in an especially simple manner.

It also proves to be an advantage for a safety bearing carrier to bearranged around the outer ring for attaching the safety bearing in themachine, with the outer ring having a recess on its side facing towardsthe safety bearing carrier in which at least a part of the sensor isarranged. The sensor can be arranged in an especially simple manner inthe recess.

Furthermore it proves to be an advantage for the physical variable to bepresent in the form of the distance between outer ring and inner ring.The distance between outer ring and inner ring represents a normalphysical variable of a safety bearing which changes as wear on thebearing increases.

In this context it proves to be an advantage for at least part of thesensor to be arranged inside the outer ring. The sensor can be arrangedin an especially simple manner inside the outer ring.

It also proves to be an advantage for roller bearings to be arrangedbetween outer ring and inner ring or for the inner ring to slidedirectly in the outer ring. These embodiments represent normalembodiments of the safety bearing.

It further proves to be an advantage for the physical variable to betransmitted via a data link to a computer arranged remotely from themachine. This makes it possible to monitor the safety bearing remotely.

It also proves to be an advantage for the physical variable to be ableto be compared to a target variable or to a variable derived from thephysical variable and if the deviation of the physical variable or ofthe derived variable from the target variable exceeds a threshold value,for a warning message to be able to be generated. This makes automaticdetection of a worn safety bearing possible and for exampleautomatically informs the user of the machine or service personnel whenthe bearing is worn.

The machine can be embodied for example as an electric motor orgenerator or compressor or as a turbine. The machine can especially beembodied as a wind power generator.

BRIEF DESCRIPTION OF THE DRAWING

Exemplary embodiments of the invention are shown in the drawing and aredescribed in greater detail below. The figures show:

FIG. 1 a schematic diagram of an inventive machine with a safetybearing,

FIG. 2 a sectional view of the safety bearing and of the bearing carrierwithin the context of an exemplary embodiment of the invention,

FIG. 3 a sectional view of the safety bearing and of the bearing carrierwithin the context of a further exemplary embodiment of the invention,

FIG. 4 a safety bearing and a safety bearing carrier within the contextof a further exemplary embodiment of the invention,

FIG. 5 a flowchart of the inventive method,

FIG. 6 a first embodiment of the sensor film,

FIG. 7 a second embodiment of the sensor film,

FIG. 8 a safety bearing and a safety bearing carrier within the contextof a further exemplary embodiment of the invention and

FIG. 9 a schematic diagram of a controller of the machine.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a schematic diagram showing the elements of importance forunderstanding of the invention of a machine 12 which is embodied withinthe context of the exemplary embodiment as an electric motor. Otherelements of the machine, such as rotor yoke etc. for example, are notshown in FIG. 1 for reasons of clarity and since they are not ofsignificance for understanding the invention. The machine 12 has arotatably arranged rotor shaft 1 supported by means of a magneticbearing 6 which rotates around an axis of rotation R when the machine 12is operating.

A magnetic bearing 6 holds the rotor shaft 1 by means of a regulatedmagnetic field suspended in an air gap 21. For this purpose the magneticbearing has as its major elements coils for generating the magneticfield.

As well as the magnetic bearing 6 the machine 12 has a safety bearing 14which catches the rotor shaft 1 if the magnetic bearing 6 fails, whensaid bearing falls into the safety bearing 14 and the bearing takes overthe support of the rotor shaft 1 until the rotor shaft 1 comes to astandstill. Such a failure of the magnetic bearing 6 can for exampleoccur during a failure of the power supply of the machine 12 and thus ofthe magnetic bearing 6.

The safety bearing 14 has an outer ring 3 and an inner ring 2, arrangedrotatably in relation to the outer ring 3. To attach the safety bearing14 in the machine 12 a safety bearing carrier 4 is arranged around theouter ring 3 wherein, in the context of the exemplary embodiment, thesafety bearing carrier 4 is embodied in the shape of a ring and isarranged around the outer side of the outer ring 3. To attach the safetybearing 14 the safety bearing 14 is introduced into the safety bearingcarrier 4.

The machine 12 also has a stationary machine housing 28 to which thesafety bearing carrier 4 is attached, wherein the attachment betweensafety bearing carrier 4 and the machine housing 28 is not shown in FIG.1 for reasons of clarity.

An air gap 22, which is slightly wider than the air gap 21, is arrangedbetween the inner ring 2 and the rotor shaft 1. With the magneticbearing 6 switched on and functioning normally, the inner ring 2 of thesafety bearing 14 thus has no contact with the rotor shaft 1. If themagnetic bearing 6 fails, as a result of a power outage for example, therotor shaft 1 falls into the safety bearing 14 and there is mechanicalcontact between the inner ring 2 and the rotor shaft 1 rotating duringoperation of the machine 12, especially rotating rapidly, which oftenleads to more rapid wear of the safety bearing 14.

To control and regulate the magnetic bearing 6 the machine 12 has acontrol device 7 which is connected to the magnetic bearing 6 byelectrical leads 8 and electrical leads 9, which are shown within thecontext of the schematic representation in FIG. 1 as dashes. Thecontroller 7 regulates the magnetic fields generated by the magneticbearing 6 such that the rotor shaft 1 is held floating in the air gap 21by the magnetic field. The controller 7 contains the control andregulation functionalities necessary for this purpose. Furthermore thecontroller 7 contains current converters for controlling the magneticbearing 6. The measurement devices and feedback paths to the controller7 necessary to measure the gap between magnetic bearing 6 and rotorshaft 1 for the regulation of the magnetic field are not shown in FIG. 1for reasons of clarity and because they are not of significance forunderstanding the invention.

In accordance with the invention the machine 12 has a sensor 5 whichmeasures a physical variable G of the safety bearing. The measuredphysical variable G is read in in this case within the context of theexemplary embodiment by the controller 7. The physical variable can forexample be present in the form of the temperature of the safety bearingor in the form of a force F occurring between the outer ring 3 of thesafety bearing 14 and the safety bearing carrier 4 or in the form ofoscillations of the safety bearing or in the form of a pressureoccurring between outer ring and the safety bearing carrier or in theform of the distance between outer ring and inner ring. In this way thesensor can be used for example as a temperature sensor for measuring thetemperature of the safety bearing or as a force sensor for measuring theforce F occurring between outer ring and safety bearing carrier or as anoscillation sensor for measuring oscillation of the safety bearingcarrier or as a pressure sensor for measuring the pressure occurringbetween outer ring and safety bearing carrier or as a displacementsensor for measuring the distance between outer ring and inner ring.Within the context of the exemplary embodiment according to FIG. 1 thesensor 5 is embodied here as a force sensor and measures the force Foccurring between outer ring 3 and the safety bearing carrier 4. Thesensor 5 is arranged between safety bearing carrier 4 and outer ring 3.The sensor 5 is arranged in this way in the force flow between thesafety bearing 14 and the safety bearing carrier 4.

Within the context of the exemplary embodiment according to FIG. 1, thesensor 5 is embodied flat, as shown in FIG. 6 and FIG. 7, and isarranged embedded into a film 29 (see FIG. 6) or arranged on a film (seeFIG. 7). The sensor 5 thus forms a so-called sensor film together withthe film 29. Within the context of the exemplary embodiment the sensor 5measures the force F. The sensor film in this case is not showntrue-to-scale in the diagram according to FIG. 1 for reasons of claritybut is shown significantly thicker than it is in reality. The measuredforce F is then, as already described above, read in within the contextof the exemplary embodiment by the controller 7. However the sensor filmcould also, with a corresponding embodiment of the sensor, measure thetemperature, the oscillations or the pressure for example, withdifferent sensors also able to be embedded in a common film or arrangedon a common film for measuring different physical variables (e.g.temperature sensor and force sensor).

FIG. 2 shows a sectional view of the safety bearing and of the bearingcarrier 4, with the same elements being labeled with the same referencecharacters in FIG. 2 as in FIG. 1. In the embodiment of the invention inaccordance with FIG. 1 and FIG. 2 the inner ring 2 slides directly inthe outer ring 3. The safety bearing is thus embodied as a slidingcontact bearing.

The exemplary embodiment shown in FIG. 3 corresponds in its basicstructure essentially to the embodiment described in FIG. 1 and FIG. 2.The same elements are thus provided in FIG. 3 with the same referencecharacters as in FIG. 1 and FIG. 2. The only significant difference isthat, in the exemplary embodiment in accordance with FIG. 3, rollerbearings 27 are arranged between the outer ring 3 and inner ring 2,which are embodied within the context of the exemplary embodiment in theform of ball bearings. Within the context of the exemplary embodiment inaccordance with FIG. 3 the safety bearing is thus embodied as a rollerbearing.

A further embodiment of the invention is shown in FIG. 4 within thecontext of a schematic diagram, which essentially corresponds to theembodiment according to FIG. 3, with the same elements being providedwith the same reference characters as in FIG. 3. The only significantdifference from the embodiment in accordance with FIG. 3 is that thesensor 5 is not embodied in the form of a sensor film, but as aconventional force sensor. On its side facing towards the safety bearingcarrier 4, the outer ring 3 has a recess 13, in which at least a part ofthe sensor 5 is arranged. The sensor 5 in this case is arranged on itsupper side on the safety bearing carrier 4 and on its lower side on theouter ring 3. Thus the sensor 5 is arranged between outer ring 3 andsafety bearing carrier 4 and measures the force occurring between outerring and the safety bearing carrier 4. The sensor 5 can however also beembodied as a temperature sensor for measuring the temperature of thesafety bearing or as an oscillation sensor for measuring oscillations ofthe safety bearing or be present as a pressure sensor for measuring thepressure occurring between outer ring and safety bearing carrier. In anembodiment of the sensor 5 as temperature sensor or as oscillationsensor the sensor 5 is preferably arranged completely in the recess 13,i.e. it does not stand proud of the recess 5, as shown in FIG. 4. Itshould be noted here that the sensor can also be embodied as a sensorfilm in this embodiment, with at least part of the sensor of the sensorfilm or the sensor film being able to be arranged completely in therecess 13.

A further embodiment of the invention is shown in FIG. 8 within thecontext of a schematic diagram which essentially corresponds to theembodiment in accordance with FIG. 3, with the same elements beingprovided with the same reference characters as in FIG. 3. The onlysignificant difference compared to the embodiment in accordance withFIG. 3 is that the sensor 5 is not embodied in the form of a sensor filmbut as a conventional displacement sensor for measuring the distance dbetween outer ring 3 and inner ring 2. Within the context of thisexemplary embodiment the outer ring 3 has a recess 13′ on its innerside, wherein in the exemplary embodiment a part of the sensor 5 isarranged in the recess 13′ and a part of the sensor 5 is arranged insidethe outer ring 3. The sensor 5 can however also be arranged completelyinside the outer ring 3. The sensor embodied in this exemplaryembodiment as a displacement sensor 5 can however also be arranged atanother point.

The leads which lead away from the sensor 5 for transmission of thephysical variable are preferably routed through the safety bearingcarrier 4, a feature which is not shown in the figures the reasons ofclarity. Naturally, with an embodiment of the safety bearing as asliding contact bearing in accordance with FIG. 1 and FIG. 2, the outerring 3, like the embodiment of the invention in accordance with FIG. 4and FIG. 8, can have a recess in which at least a part of the sensor 5or the sensor 5 is completely arranged.

FIG. 5 shows the inventive method for monitoring the state of the safetybearing in the form of a flowchart. In this flowchart, when the methodis carried out, in a first step 15 the magnetic bearing 6 is switchedoff, then in a second step 16 the rotor shaft 1 is moved rotationallywith a defined course of movement, with a physical variable G of thesafety bearing 14 being measured and stored by means of the sensor 5,after which in a third step 17 the measured physical variable iscompared with the target variable and if necessary in a fourth step 18 awarning message is generated if the deviation of the measured physicalvariable G from the target variable exceeds a threshold value. Theexecution of the method is explained in greater detail below. In a firststep 15 the magnetic bearing 6 is switched off, preferably with therotor shaft 1 at a standstill. Within the context of the exemplaryembodiment the magnetic bearing 6 is switched off by the controller 7.The rotor shaft 1 then falls into the safety bearing 14 and is caught bythe latter. Subsequently in a step 16 the rotor shaft 1 is movedrotationally with a defined course of movement. Such a defined course ofmovement can for example consist of the rotor shaft 1 rotating slowly ata predetermined constant speed over a predetermined period in the safetybearing 14. For this purpose the rotor shaft is driven accordingly bythe machine 12, an action which is controlled by a higher-rankingcontroller 23 (see FIG. 1 and FIG. 9) which can for example be embodiedin the form of a numerical control of the machine 12. To this end thehigher-ranking controller 23, as shown schematically in FIG. 9, controlsthe machine 1 embodied within the context of the exemplary embodiment asan electric motor via a drive device 30, which comprises a regulationdevice and a current converter needed to supply energy to the machine12. The drive device 30 is connected via electrical leads 31, which areshown schematically in the form of a solid line, to the machine 1. Toregulate the speed of the rotor shaft 1 the angle of rotation W of therotor shaft 1 is transferred from a measurement device integrated in themachine 12 to the drive device 30. The speed at which the rotor shaft 1is to turn is prespecified in this case to the drive device 30 by thehigher-ranking controller 23 via a data connection 32 (e.g. data bus).

During the execution of the defined course of movement in step 16 (seeFIG. 5) a physical variable, such as the temperature of the safetybearing or the force F occurring between the outer ring and the safetybearing carrier or the oscillations of the safety bearing or thepressure occurring between outer ring and the safety bearing carrier orthe distance between outer ring and inner ring for example, is measuredand stored, with the physical variable preferably being stored in thecontroller 7.

The measured physical variable can subsequently be read out andevaluated by a user locally at the machine for example. The temporalsequence of the physical variable can be shown for this purpose in theform of a diagram on an operating device 24 of the machine for example.To this end the operating advice 24 is linked to the control device 7via the higher-ranking controller 23 for transmission of data, which isshown by the arrows 25 and 26. The physical variable, which is presentin the form of temporally consecutive measured values, is transferredfrom the control device 7 to the operating device 24 and evaluated thereby an operator. For this purpose the sensor transfers the measuredvalues to the control device 7, preferably at constant intervals.

As an alternative or in addition to this, within the context of theexemplary embodiment, the control device 7 is connected via the Internet10 and/or e.g. via a bus system to a computer 11 arranged remotely fromthe machine 12 for transmission of data, which is shown by the twoarrows 19 and 20 in FIG. 1. The Internet or a bus system are examples oftypical data connections in this case. The physical variable, i.e.expressed more precisely the measured values, can be transferred forthis purpose from the control device 7 to the computer 11 and evaluatedthere for example by service personnel. As an alternative, instead oftransferring the data directly from the control device 7 via theInternet 10 to the computer 11, this data can also be initiallytransferred via the connection 25 from the control device 7 to thehigher-ranking controller 23 and transferred from there via the Internet10 for example to the computer 11, which is shown in FIG. 1 by means ofa dashed-line arrow.

In addition however an automatic evaluation of the measured physicalvariable can also be carried out in the control device 7 or in thehigher-ranking controller 23 or in the computer 11. To this end, in astep 17 the measured physical variable, which is present in the form oftemporally consecutive measured values, is compared with the targetvariable and if the deviation between measured physical variable andtarget variable exceeds a threshold value, a warning message isgenerated in a step 18. The physical variable, as already described, ispresent in this case in the form of temporally consecutive measuredvalues. The target variable can be determined for example in that, witha newly installed safety bearing with the magnetic bearing switched off,the rotor shaft 1 is moved rotationally with a defined course ofmovement and the physical variable is measured here and stored as thetarget variable. The target variable is thus present as a rule in theform of temporally consecutive target values, preferably determined byone-off measurement.

If the deviation between measured physical variable and target variableexceeds a threshold value, in a step 18, depending on where theevaluation is implemented, a warning message is generated by thehigher-ranking controller 23 or the control device 7 or the computer 11.The deviation can be defined for example by the amount of the differenceof the measured values from the target values being determined. Thewarning message notifies the operator on the spot and/or servicepersonnel remote from the machine about a heavily worn safety bearingwhich must be replaced. The measured physical variable and the targetvariable are present in such cases, as already stated, generally in theform of temporal sequences. The wear of a safety bearing thus has thegeneral effect, during the defined course of movement, of thetemperature of the safety bearing increasing more rapidly and/or highertemperature values being reached than with an unworn bearing.Furthermore it is generally the case for a worn safety bearing comparedto an unworn safety bearing, during execution of the defined course ofmovement, that the measured force and the measured pressure which occurbetween outer ring of the safety bearing and the safety bearing carrierand/or the distance between outer ring and inner ring are changed and/orunusual or greater oscillations of the safety bearing occur. Within thecontext of the exemplary embodiment in such cases the force F measuredby the sensor 5 is evaluated as the physical variable as described aboveand if necessary a warning message is generated.

It should be noted at this point that, for measuring the physicalvariable, the machine can naturally have not only a single, but also anumber of sensors which measure the physical variable, e.g. at differentpoints and/or in different directions (e.g. in the case of a force oroscillations). The evaluation can take place separately for eachmeasurement signal of the sensors for example.

It should also be noted at this point that the force occurring betweenouter ring 3 and the safety bearing carrier can not only act in theradial direction, as indicated in FIG. 1, but also in the tangentialdirection, i.e. in the direction of the rotational movement of the rotorshaft 1. The sensor 5 can also be embodied so that it measures the forceacting in the direction of the rotational movement of the rotor shaft 1.Within the controller 7, e.g. a variable derived from the physicalvariable can be determined and this can then be compared to a targetvariable, wherein, if the deviation between the variable derived fromthe physical variable and the target variable exceeds a threshold value,a warning message is generated. The variable derived from the physicalvariable can be present in the form of torque for example, which isdetermined from the force acting in the direction of the rotationalmovement of the rotor shaft 1 and the distance of the sensor from theaxis of rotation R by multiplication of the two variables. The targetvariable is correspondingly present in this case in the form of atorque. Accordingly variables can also be derived from other physicalvariables (e.g. temperature, oscillations) and compared with acorresponding target variable for evaluation. Furthermore the force Fcan be derived for example from the measured distance d between outerring 3 and inner ring 2.

It should also be noted at this point that the machine can of coursealso have a number of sensors for measuring different physicalvariables. Thus for example the machine can also simultaneously have asensor which measures the temperature of the safety bearing and/or asensor which measures the force occurring between outer ring and thesafety bearing carrier and/or a sensor which measures the oscillationsof the safety bearing and/or a sensor which measures the pressureoccurring between outer ring and the safety bearing carrier and/or asensor which measures the distance between outer ring and inner ring.The respective sensor in this case can be arranged corresponding to thesensor 5, e.g. on the side of the outer ring facing towards the safetybearing carrier and can be arranged at least partly or completely in therecess 13 of the outer ring 3 and/or between outer ring and safetybearing carrier and/or at least partly in the inside of the outer ring3. The measured physical variables are preferably evaluated in parallelin such cases, with each measured physical variable being evaluated asdescribed in FIG. 5 and the associated description for example.

The inventive method makes preventive maintenance possible. Theinventive method also enables the safety bearing to be monitoredremotely by service personnel without service personnel having to go tothe machine on site for this purpose.

What is claimed is:
 1. A method for monitoring a state of a safetybearing of a rotor shaft of a machine, wherein the rotor shaft issupported by a magnetic bearing, the safety bearing having an outer ringand an inner ring arranged for rotation with respect to the outer ring,the method comprising the steps of: catching a rotor shaft of themachine with the safety bearing when the magnetic bearing of the machinefails, switching the magnetic bearing off for monitoring the state ofthe safety bearing, rotating the rotor shaft with the machine undercontrol of a higher-ranking controller by using a defined motionsequence, and measuring a physical variable of the safety bearing with asensor.
 2. The method of claim 1, further comprising the steps of:comparing the measured physical variable or a variable derived from themeasured physical variable with a target variable, and generating awarning message, if a deviation of the physical variable or the derivedvariable exceeds a threshold value.
 3. The method of claim 1, whereinthe safety bearing is attached in the machine with a safety bearingcarrier arranged around the outer ring, and wherein the sensor isarranged on a side of the outer ring facing the safety bearing carrier.4. The method of claim 1, wherein the physical variable comprises avariable selected from the group consisting of a temperature of thesafety bearing, a force between the outer ring and the safety bearingcarrier, oscillations of the safety bearing, and a pressure betweenouter ring and the safety bearing carrier.
 5. The method of claim 1,wherein the sensor is arranged between the outer ring and the safetybearing carrier.
 6. The method of claim 1, wherein the sensor has a flatconfiguration and is embedded in or arranged on a film.
 7. The method ofclaim 3, wherein at least a part of the sensor is arranged in a recessdisposed on the side of the outer ring facing the safety bearingcarrier.
 8. The method of claim 1, wherein the physical variable is adistance between the outer ring and the inner ring.
 9. The method ofclaim 8, wherein at least part of the sensor is arranged inside theinner ring.
 10. The method of claim 1, wherein roller bearings arearranged between the outer ring and inner ring, or wherein the innerring slides directly in the outer ring.
 11. The method of claim 1,wherein the physical variable is transmitted via a data link to acomputer arranged remote from the machine.
 12. The method of claim 1,wherein the machine is embodied as an electric motor, a generator, acompressor, or a turbine.
 13. The method of claim 17, wherein themachine is embodied as a wind power generator.
 14. A machine, comprisinga rotor shaft, a magnetic bearing supporting the rotor shaft andconfigured to be switched off, a safety bearing comprising an outer ringand an inner ring arranged for rotation relative to the outer ring, thesafety bearing constructed to catch the rotor shaft when the magneticbearing fails, wherein when the magnetic bearing is switched off formonitoring a state of the safety bearing, the rotor shaft is rotatedwith the machine under control of a higher-ranking controller by using adefined motion sequence, and a sensor measuring a physical variable ofthe safety bearing.
 15. The machine of claim 14, wherein the measuredphysical variable or a variable derived from the measured physicalvariable is compared with a target variable, and a warning message isgenerated, if a deviation of the physical variable or the derivedvariable exceeds a threshold value.